The present invention relates to a non-aqueous electrolyte secondary battery comprising a positive electrode which contains manganese-contained complex oxide containing lithium (Li) and manganese (Mn), and nickel-contained complex oxide containing lithium (Li) and nickel (Ni).
Recently, in accordance with development in electronic technology, a number of small-size portable electron devices such as VTRs (video tape recorder) with a built-in camera, cellular phones and laptop computers have come into wide use, and miniaturization and lightening of the devices have been the subject. As a portable electric source used for the devices, small and light batteries with high energy density, specifically secondary batteries, have been at the stage of development. A large expectation has been put on lithium-ion secondary batteries since the battery has a higher energy density compared to lead batteries or nickel-cadmium batteries of the related art using an aqueous liquid electrolyte.
In lithium-ion secondary batter of the related art, it is known to use a carbonaceous material for a negative electrode and lithium-contained complex oxide such as lithium-cobalt complex oxide, lithium-manganese complex oxide and lithium-nickel complex oxide for a positive electrode. The battery using lithium-cobalt complex oxide as a material for a positive electrode has been widely put in practical use since it is most excellent in terms of the battery capacity, cost, thermal stability and the like in each respect. On the contrary, the one using lithium-manganese complex oxide and the ones using lithium-nickel complex oxide are excellent in terms of cost for the source material and a stable supply even though the former has a small battery capacity and insufficient preservation characteristic at a high temperature, and the latter has a relatively low thermal stability. The two-types have been studied for future use. For example, recently, a technique is disclosed (see Japanese Patent Application Laid-open Hei 8-45498) in which the defects of both types are supplied by mixing lithium-manganese complex oxide and lithium-nickel complex oxide. Thereby, expansion/contraction of the positive electrode is thereby suppressed at the time of charging/discharging, and the charging/discharging cycle characteristic is improved.
However, the secondary battery using the mixture of lithium-manganese complex oxide and lithium-nickel complex oxide has a shortcoming that the characteristic is deteriorated when preserved at a high temperature of, for example, 45° C. to 60° C. Specifically, the battery, when used for information terminal for cellular phones or the like, requires capacity with heavy load (in a state of large current density) and high termination voltage. However, a sufficient capacity cannot be obtained after preservation at a high temperature. Also, in the above-mentioned secondary battery, a sufficient charging/discharging cycle characteristic cannot always be obtained depending on the particle diameter of lithium-manganese complex oxide and lithium-nickel complex oxide. Furthermore, in order to satisfy the recent demand for high energy density, it is necessary to obtain a larger capacity.